The present invention relates to an improvement in a panel key switch, particularly suitable for use in a keyboard on a telephone set.
FIG. 1A and FIG. 1B show conventional panel key switches. FIG. 2A and FIG. 2B show an arrangement of panel key switches in a keyboard. As illustrated, key tops 1 are provided on a front panel 2, which holds the key tops 1 and regulates the movement of the key tops 1 so that the key tops 1 can be manipulated properly. Rubber contacts 3 and corresponding parts of a printed circuit board 4 form switch circuits for the respective key tops 1. Each of the switch circuits is closed when the corresponding key top 1 is pressed. Light-emitting diodes 4a are mounted on a printed circuit board 4 to illuminate the key tops 1 from the rear or the inside of the front panel.
Characters are printed on the surface 1g of each key top 1. The printed characters are observed under sun light or other illumination. To enable observation of the characters in the dark, light-emitting diodes 4a are turned on to illuminate the rear surface of the key tops 1 so that the character areas 1b look dark and the background area 1d look light (or bright). To this end, the key top 1 must be made of a translucent material. If the key top 1 is made of a completely transparent material, the brightness is not uniform throughout the surface 1g of the key top 1, so it is difficult to see the characters.
Because the prior-art self-illuminating panel switch is formed as described above, the printed characters are contacted by a finger or the like. It is therefore necessary that the ink forming the printed characters is firmly attached to the surface of the key top, or is protected by a coating formed over it, or by a special configuration of the surface of the panel. Moreover, the surface 1g of the key top 1 must be flat to facilitate the printing. These impose limitations on the fabrication process and the design.
The above-described problem is also present when characters are printed with white or light color on a dark background.